Flash multiple-pass write with accurate first-pass write

ABSTRACT

An indication to store a data value in Flash memory is received. An accurate coarse write is performed on the Flash memory, including by: storing a first voltage level in the Flash memory and setting a configuration setting of the Flash memory to a first setting. The first voltage level, when interpreted using the configuration setting at the first setting, corresponds to the data value. A fine write is performed on the Flash memory, including by: storing a second voltage level in the Flash memory and setting the configuration setting of the Flash memory to a second setting. The second voltage level, when interpreted using the configuration setting at the second setting, corresponds to the data value.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/422,774 (Attorney Docket No. LINKP089), entitled FLASHMULTIPLE-PASS WRITE WITH ACCURATE FIRST-PASS WRITE filed Mar. 16, 2012which is incorporated herein by reference for all purposes, which claimspriority to U.S. Provisional Patent Application No. 61/471,633 (AttorneyDocket No. LINKP089+) entitled DATA RECOVERY FROM UNFINISHEDMULTIPLE-PASS WRITE DURING POWER INTERRUPTION filed Apr. 4, 2011 whichis incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Multiple-pass write to Flash memory is a technique in which a write isdivided into multiple passes or stages. The first pass is sometimesreferred to as a coarse write, with later writes referred to as finewrites. Multiple-pass writes are popular because it mitigatesinterference at one write line caused by writing to an adjacent writeline in Flash memory. It would be desirable if improved techniques couldbe developed for multiple-pass writes, for example which reduce bususage and/or overhead memory associated with existing techniques (e.g.,overhead in the Flash memory itself or in a separate random accessmemory (RAM)), and/or which permit recovery of the data in the eventpower is lost between a coarse write and a fine write.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a diagram showing an embodiment of a Flash storage system anda multiple-pass write sequence associated with the Flash storage system.

FIG. 2 is a flowchart illustrating an embodiment of a multiple-passwrite process with an accurate coarse write.

FIG. 3 is a diagram showing an embodiment of voltage distributions foran accurate coarse write and a fine write where level placement isadjusted.

FIG. 4 is a diagram showing an embodiment of voltage distributions foran accurate coarse write and a fine write where a correct data value isnot necessarily guaranteed.

FIG. 5 is a diagram showing an embodiment of different ramp windows usedfor accurate coarse writes and fine writes.

FIG. 6 is a diagram showing various embodiments of program pulses.

FIG. 7 is a flowchart illustrating an embodiment of a process foradjusting one or more configuration settings associated with Flashmemory.

FIG. 8 is a diagram showing an embodiment of an interface between aFlash controller and a Flash memory used to change one or moreconfiguration settings of a Flash memory during an accurate coarse writeand a fine write.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

FIG. 1 is a diagram showing an embodiment of a Flash storage system anda multiple-pass write sequence associated with the Flash storage system.In the example shown, Flash controller 150 writes data to and reads datafrom Flash memory 152. Flash controller 150 is configured to performmultiple-pass write, specifically (at least in this example) a 2-passwrite where an accurate coarse write is performed prior to a fine write.For example, write lines 100-103 are included in Flash memory 152. Anaccurate coarse write is performed on write line 100 at 106, followed byan accurate coarse write (108) to write line 101. Next, a fine write(110) is performed on write line 100 and an accurate coarse write isperformed at 112 on write line 102. At 114, a fine write is performed onwrite line 101, followed by an accurate coarse write at 116 to writeline 103. This is followed by a fine write at 118 to write line 102.

Writing to write lines 100-103 as shown in this exemplary sequencemitigates interference between write lines due to the writing process.For example, writing to write line 100 causes some additional charge tobe added to one or more voltages stored in write line 101. In Flashmemory, data is stored as voltage levels, so an increase in voltagelevel (e.g., due to a write to an adjacent location) may cause an error(e.g., if the noise is enough to push the stored voltage into a nextvoltage range and an error correction code does not fix the error).

Flash controller 150 is configured to perform an accurate coarse write.An accurate coarse write is a coarse write in a multi-pass write inwhich the correct value is knowingly and/or deliberately written (e.g.,so that if read back, the value would be interpreted as the correctvalue). For example, in a 2-bit Flash storage scenario where the levelsare L0-L3 (e.g., corresponding to 00, 01, 10, 11, or some othermapping), and where the level L0 corresponds to voltages below athreshold v₀₁, an accurate coarse write (which is part of a multi-passwrite which is writing the level L0 to some write line) would program avoltage that is less than v₀₁. An accurate coarse write may includeadjusting or otherwise setting a configuration setting associated withFlash storage to some setting or value for the accurate coarse write(e.g., where that configuration setting has some other value for acorresponding fine write). In contrast, some other systems which do notperform an accurate coarse write do not necessarily know and/or carewhat value is being written during a coarse write. In some embodiments,an accurate coarse write does not necessarily guarantee that the correctvalue would be read back as the correct value all of the time orabsolutely, but rather attempts to write or program the correct value toa sufficient or satisfactory degree (e.g., for some statisticaloutliers, the stored value would not be interpreted as the correct valueif read back but the outliers are acceptably infrequent or unlikely).

One advantage to an accurate coarse write is that a Flash system is ableto recover even if there is an interruption between a coarse write to agiven write line and the subsequent fine write to that write line. Forexample, if there is a power loss between the coarse write at 108 andbefore the fine write at 110, the values stored in write lines 100 and101 are able to be read back as their correct values since coarse writes106 and 108 are accurate coarse writes. Another advantage is a possiblereduction in overhead storage requirements. Some other systems which donot perform accurate coarse writes use overhead memory to store acorrect value, for example on a separate random access memory (RAM)semiconductor device. In one exemplary system, the correct value isfirst stored in the RAM (e.g., at some point prior to a fine write), andis then read back from the RAM with the read-back value being usedduring the fine write. This requires additional components (e.g., theRAM) in order to store the data for multiple NAND device programs. Thetechnique described herein does not prevent usage of a RAM to speed upthe fine write without reading the accurate coarse write from the NAND.Some other systems store a correct value in the Flash memory itself(e.g., in single-level cells (SLC) of Flash memory 152) which reducesusable capacity. A Flash controller configured to perform accuratecoarse writes may not necessarily require such overhead storage.

In some embodiments, Flash controller 150 and/or Flash memory 152 areimplemented in hardware, for example as a semiconductor device such asan application-specific integrated circuit (ASIC), a field-programmablegate array (FPGA), or an embedded microprocessor such as an AdvancedRISC Machine (ARM) embedded core.

Although this example and other examples described herein may refer to acoarse write or to a 2-pass write, the techniques described herein maybe for multi-pass writes with any number of passes, such as a 3-passwrite system.

FIG. 2 is a flowchart illustrating an embodiment of a multiple-passwrite process with an accurate coarse write. In some embodiments, theprocess is performed by a Flash controller, such as Flash controller 150in FIG. 1.

At 200, an indication to store a data value in Flash memory is received.For example, a driver or other upper level entity may instruct a Flashcontroller to store data in Flash memory, and the system is configuredto perform writes in multiple passes in order to mitigate interference.At 201, an accurate coarse write is performed on Flash memory, includingby storing a first voltage level in the Flash memory and setting aconfiguration setting of the Flash memory to a first setting, whereinthe first voltage level, when interpreted using the configurationsetting at the first setting, corresponds to the data value. Forexample, if the data value to be stored corresponds to level L1 from aset of possible levels L0-L3 (i.e., a 2-bit system), after the accuratecoarse write at 201 but before the fine write at 206, if read back, thelevel stored in write line of interest would be interpreted as L1 (e.g.,based on the configuration set at 201). Some examples of Flash memoryconfigurations which may be set at 201 include ones related to levelplacement, program pulses, and/or ramp windows.

Steps 202 and 204 are optional and are not performed in someembodiments. At 202, Flash memory is read. It is determined at 204 if itis a correct value (e.g., if the level L1 was intended to be written at201, was L1 returned at 202?). If so, at 206, a fine write is performedon Flash memory, including by storing a second voltage level in Flashmemory and setting the configuration setting of the Flash memory to asecond setting, wherein the second voltage level, when interpreted usingthe configuration setting at the second setting, corresponds to the datavalue. For example, if the configuration setting is associated withlevel placement, the second voltage level stored at 206 may be greaterthan the first voltage level, but the setting of the level placement(s)is such that the second voltage level stored corresponds to the correctlevel (e.g., L1 in the example above). If not, an accurate coarse writeis performed again at 201.

In one example of performing a read at 202 and determining if it is acorrect value at 204, in FIG. 1, write line 102 in FIG. 1 is read backafter the accurate coarse write at 112. In some embodiments where a readand check is performed, at most one accurate coarse write is unverifiedat a time. This may minimize an amount of memory required to storecorrect values for unverified write lines (e.g., for the comparison at204 between the correct value and what was read back). If so performed,repeating a coarse write at 201 (e.g., after it is determined at 204that the value read back at 202 is not the correct value) is performedin a variety of ways, for example depending upon how a particularaccurate coarse write is implemented and/or what controls or interfacesare exposed by a particular Flash memory device. In some embodiment, ifthe stored voltage is lower than a voltage range corresponding to thecorrect value, one or more program pulses is performed to bring thestored voltage up to a voltage that corresponds to the correct datavalue. In some cases, if the stored voltage level is too high and it ispossible to adjust the level placement, repeating an accurate coarsewrite at 201 includes adjusting a level placement (e.g., withoutperforming any additional program pulses) so that the stored value fallswithin the new level placements corresponding to the correct value.

As shown in FIG. 1, fine writes may be interleaved with accurate coarsewrites. For clarity, the steps in the example process shown here are tothe same write line (e.g., the accurate coarse write at 201 and the finewrite at 206 are to the same write line). Conceptually, multipleversions of the example process shown may be operating simultaneously,where each process is associated with a particular write line. Forexample, there might be one process running for write line 100 in FIG.1, another process running for write line 101 in FIG. 1, and so on.

As part of performing an accurate coarse write, one or moreconfiguration settings associated with Flash memory may be adjusted. Thefollowing figures describe some example configuration settings andexemplary values for those configuration settings during an accuratecoarse write and fine write.

FIG. 3 is a diagram showing an embodiment of voltage distributions foran accurate coarse write and a fine write where level placement isadjusted. Level placement refers to the voltage ranges which define eachlevel. In diagram 300, for example, the levels L0-L3 associated with anaccurate coarse write are defined by the thresholds v₀₁, v₁₂, and v₂₃and in diagram 350 the levels L0′-L3′ associated with a fine write aredefined by the thresholds v₀₁′, v₁₂′, and v₂₃′. Each of the levels maycorrespond to a particular data value, for example levels L0 and L0′correspond to a data value of 00, levels L1 and L1′ correspond to a datavalue of 01, levels L2 and L2′ correspond to a data value of 11, andlevels L3 and L3′ correspond to a data value of 10. In this particularexample, v₀₁≠v₀₁′, v₁₂≠v₁₂′, and v₂₃≠v₂₃′, and more specifically,v₀₁<v₀₁′, v₁₂<v₁₂′, and v₂₃≠v₂₃′.

Diagrams 300 and 350 also show example voltage distributions for each ofthe levels for both the accurate coarse write and the fine write. Inthis example, the means of the voltage distributions associated with theaccurate coarse write are less than the means of their correspondingfine write voltage distributions in 350. For example, the mean of thedistribution for L0 is less than the mean of the distribution for L0′.The widths of the distributions for the accurate coarse write are alsogreater than the distributions of their fine write counterparts (e.g.,the standard deviation (or variance) of the distribution associated withL0 is greater than the standard deviation (or variance) of thedistribution associated with L0′).

Achieving a particular voltage distribution (e.g., having a desiredmean, a desired width, a desired fall-off/shape, etc.) during anaccurate coarse write or fine write depends upon the particular Flashmemory implementation. Different Flash memory devices will exposedifferent interfaces, controls, and/or configuration settings which canbe manipulated to achieve a desired voltage distribution.

Although not shown in this example, the distribution for accurate coarsewrite 300 may have a narrower distribution than some other systems'coarse writes. For example, to ensure the accuracy of an accurate coarsewrite, it may also be necessary to control the precision of a coarsewrite and since other systems do not care about what value would be readback, those systems care about neither accuracy nor precision. In someembodiments, the width of a distribution associated with an accuratecoarse write is substantially the same as that of a distributionassociated with a fine write. For example, as more bits are stored percell, the distance between two thresholds (e.g., between v₀₁ and v₁₂)may decrease and to ensure an accurate coarse write is read back as acorrect value to a sufficient or acceptable degree, the width of adistribution for an accurate coarse write may be reduced in order toachieve this objective.

The level placements and distributions shown are merely exemplary andsome other embodiments employ other level placements and/or voltagedistributions. In some embodiments, a spacing between thresholds (e.g.,between v₀₁ and v₁₂ or between v₀₁′ and v₁₂′) varies between an accuratecoarse write and a fine write (e.g., (v₁₂−v₀₁)≠(v₁₂′−v₀₁′)). In someembodiments, one or more level placements remain the same for anaccurate coarse write and a fine write (e.g., threshold v_(oi) remainsthe same but thresholds v₁₂ and v₂₃ change).

In some applications, adjusting a level placement between an accuratecoarse write and a fine write (one example of which is shown herein) isattractive because it makes achieving an accurate coarse write easierand/or increases a degree or likelihood of a value written by anaccurate coarse write being interpreted as a correct value if read back.

In some embodiments, a storage system uses an error correction code orerror detecting code on the stored data and the techniques describedherein may be used in combination with various coding techniques. Insome embodiments, an accurate coarse write does not necessarilyguarantee that a value will be interpreted as a correct value all of thetime, for example, even after any error correction decoding or errordetection decoding.

FIG. 4 is a diagram showing an embodiment of voltage distributions foran accurate coarse write and a fine write where a correct data value isnot necessarily guaranteed. In the example shown, some portions of someof the voltage distributions in diagram 400 do not lie within the propervoltage ranges. For example, portion 402 of distribution 401 is belowthe threshold v₀₁ and therefore those voltages (which occur relativelyinfrequently) would incorrectly be interpreted as level L0 instead oflevel L1. However, the frequency or degree to which this occurs may beacceptable or satisfactory (e.g., in light of desired systemperformance, an expected environment of operation, mean time betweenfailures, etc.). As shown in this figure, in some embodiments anaccurate coarse write does not necessarily guarantee that a data valueor level written by an accurate coarse write will always be interpretedas the correct data value or correct level if read back.

FIG. 5 is a diagram showing an embodiment of different ramp windows usedfor accurate coarse writes and fine writes. In some embodiments, a rampwindow is adjusted, for example so that a first ramp window is used foran accurate coarse write and a second ramp window is used for a finewrite. A ramp window is the range of voltages which a Flash cell ispermitted or otherwise configured to store. By adjusting the rampwindow, different voltage floors and ceilings (i.e., v_(L) and v_(H))are able to be set. Voltages which are out of range or are not permittedfor a particular ramp window are shown in the shaded portion. In thisparticular example, v_(L)′ is strictly greater than v_(L) and v_(H)′ isstrictly greater than v_(H), but in some other embodiments ramp windowsmay be varied in other ways.

In some embodiments, a ramp window is adjusted to take advantage ofFlash memory's different performance characteristics over differentvoltage ranges. For example, it may be easier to achieve a tighterdistribution over a first voltage range as compared to a second voltagerange. In some embodiments, using one ramp window for an accurate coarsewrite and another for a fine write seeks to exploit the differentperformance characteristics. In some embodiments, different ramp windowsare used because it makes writing a correct value with an accuratecoarse write easier and/or increases a degree to which this occurs.

FIG. 6 is a diagram showing various embodiments of program pulses. Inthe example shown, diagrams 600-602 show various embodiments of programpulses. Program pulses are a series of one or more pulses of voltagesthat are sent to a Flash cell in order to program a particular voltagelevel. In other words, program pulses are used during a write operationto write a particular voltage level to Flash memory. Each voltage levelin a cell may have a different set of program pulses; for clarity, theexamples shown in diagrams 600-602 correspond to the same voltage level.For example, referring back to FIG. 3, the program pulses in thisexample may all correspond to programming the level L3 (i.e., above v₂₃)in diagram 300.

In some embodiments, a first set of program pulses is used during anaccurate coarse write and a second set of program pulses is used for afine write. For example, the program pulses shown in diagram 601 areused to program accurate coarse write in and the program pulses shown indiagram 602 are used to program level fine write. Program pulseincrement steps and/or width can affect the quality of each programmingand a flash controller may choose to use different program pulsesettings to achieve a desirable result for an accurate coarse writeand/or a fine write. As shown in this example, there are a variety ofparameters or characteristics associated with program pulses that can bevaried for an accurate coarse write and a fine write. For example, thestep size increase in diagram 600 increases exponentially (i.e., theprogram pulses have voltages of a, ar, ar², ar³, . . . ) and the stepsize increase in diagram 601 is a linear rate of increase (i.e., theprogram pulses have voltages of b, m+b, 2m+b, 3m+b, . . . ). Naturally,even though two sets of program pulses may both be linear (orexponential), different linear (or exponential) rates of increase may beemployed. For example, the rate of increase (or, alternatively, stepsize) for the program pulses in diagram 602 is less than the rate ofincrease (or, alternatively, step size) for the program pulses shown indiagram 601. Another difference between the program pulses shown indiagram 600 and 601 is the number of program pulses (i.e., 3 programpulses in diagram 600 and 5 program pulses in diagram 601). In someembodiment, a duration of program pulses varies, as is shown in diagrams601 and 602. The duration of the program pulses in diagram 602 isshorter in duration than those shown in diagram 601; in some embodimentthe program pulses shown in diagram 601 are used during an accuratecoarse write and the program pulses shown in diagram 602 are used in afine write.

FIG. 7 is a flowchart illustrating an embodiment of a process foradjusting one or more configuration settings associated with Flashmemory. Referring back to the example of FIG. 1, in some embodiments,the process is performed by Flash controller 150 on Flash memory 152.

At 700 it is determined whether a level placement is to be varied. Thisis the case, for example, in the embodiment shown in FIG. 4 where afirst set of level placements is used for an accurate coarse write and asecond set of level placements is used for a fine write. Various Flashmemory manufacturers may expose different capabilities or controls andso if a level placement or other configuration setting is not to beexposed, then this decision or a corresponding decision is “No.” In someembodiments, a system which performs an accurate coarse write isconfigurable and the decision at 700 (or some other decision in thisprocess) is based at least in part on some configuration or setting(e.g., controlled by a user, firmware, or some other entity).

If it is determined at 700 that level placement is to be varied, a levelplacement is set to a first level placement for an accurate coarse writeat 702 and at 704 the level placement is set to a second level placementfor a fine write. In some embodiments, if there are back-to-back finewrites or back-to-back accurate coarse writes, then there is no need tochange a configuration setting and so a Flash controller or other entitydoes not change a configuration setting.

After setting a level placement to be a second level placement at 704 orif it is determined at 700 that level placement is not to be varied, itis determined at 706 whether a program pulse is to be varied. Asdescribed above with respect to the decision at 700, the decision at 706may be performed in a variety of ways.

If it is determined at 706 that a program pulse is to be varied, aprogram pulse characteristic is set to a first program pulsecharacteristic for an accurate coarse write at 708. At 710, the programpulse characteristic is set to a second program pulse characteristic fora fine write. In various embodiments, a program pulse characteristic setat 708 or 710 is associated with voltage, duration, step size, or anumber of pulses. See, for example, FIG. 6 which shows some examples ofprogram pulses.

If it is determined a program pulse is not to be varied at 706 or aftersetting a program pulse characteristic at 710, at 712 it is determinedwhether a ramp window is to be varied. In FIG. 5, for example, a firstramp window is used for accurate coarse writes and a second ramp windowis used for fine writes. As described above, in some embodiments thedecision at 712 depends upon a configuration (e.g., set by a user,driver, or other entity) and/or whether controls associated with a rampwindow are exposed by a Flash memory.

If it is determined at 712 that a ramp window is to be varied, a rampwindow range is set to a first range for an accurate coarse write at714. At 716, the ramp window range is set to a second range for a finewrite. In various embodiments, a voltage floor (v_(L)) and/or a voltageceiling (v_(H)) is/are adjusted at 714 and 716. If it is determined thata ramp window is not to be varied or after setting a window range at716, the process ends.

FIG. 8 is a diagram showing an embodiment of an interface between aFlash controller and a Flash memory used to change one or moreconfiguration settings of a Flash memory during an accurate coarse writeand a fine write. In embodiments where a configuration setting is onevalue for an accurate coarse write and another value for a fine write,one or more signals in configuration bus 804 are used to change thoseconfiguration settings. In this particular example, configuration bus804 includes level placement (808), ramp window range (810), and programpulse characteristic (812). Data bus 806 (which includes write enable814, read enable 816, and read/write data 818) is used to perform readand write operations, including those related to an accurate coarsewrite and a fine write.

In one example of how Flash controller 800 adjusts configurationsettings of Flash memory 802, prior to accurate coarse write 106 in FIG.1, configuration bus 804 is used to set the level placement, ramp windowrange, and/or program pulse characteristic(s) to a value for an accuratecoarse write. The configuration setting is not necessarily adjustedbetween accurate coarse writes 106 and 108 because (at least in thisexample) the configuration settings remain the same between those twowrites. Between accurate coarse write 108 and fine write 110, theconfiguration settings which are being varied are changed to their finewrite values. For example, if a ramp window is being varied, then theramp window range is changed from a coarse ramp window range to a fineramp window range.

In some embodiments, values for configuration settings are obtained bysimulation or experimentation using real Flash memory. For example, agiven Flash memory device may expose a certain set of configurationsettings. Multiple simulations or experiments may be performed, forexample sweeping the configuration settings to exhaustively test allcombinations. In some embodiments, multiple writes and read backs areperformed for each combination of possible values in order to obtain aset of voltage distributions (e.g., similar to those shown in FIGS. 3and 4), each distribution of which corresponds to a particularcombination of possible values for the exposed configuration settings.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system, comprising: a Flash controllerconfigured to: receive an indication to store a data value in a Flashmemory; perform an accurate coarse write on the Flash memory, includingby: storing a first voltage level in the Flash memory; and setting aconfiguration setting of the Flash memory to a first setting, whereinthe first voltage level, when interpreted using the configurationsetting at the first setting, corresponds to the data value; and performa fine write on the Flash memory, including by: storing a second voltagelevel in the Flash memory; and setting the configuration setting of theFlash memory to a second setting, wherein the second voltage level, wheninterpreted using the configuration setting at the second setting,corresponds to the data value; and is the Flash memory.
 2. The system ofclaim 1, wherein: the Flash controller is configured to set theconfiguration setting of the Flash memory to the first setting,including by: setting a level placement associated with the Flash memoryto be a first level placement for the accurate coarse write; and theFlash controller is configured to set the configuration setting of theFlash memory to the second setting, including by: setting the levelplacement associated with the Flash memory to be a second levelplacement for the fine write, wherein the first level placement isdifferent from the second level placement.
 3. The system of claim 2,wherein the first level placement is strictly less than the second levelplacement.
 4. The system of claim 1, wherein: the Flash controller isconfigured to set the configuration setting of the Flash memory to thefirst setting, including by: setting a program pulse characteristicassociated with the Flash memory to be a first program pulsecharacteristic for the accurate coarse write; and the Flash controlleris configured to set the configuration setting of the Flash memory tothe second setting, including by: setting the program pulsecharacteristic associated with the Flash memory to be a second programpulse characteristic for the fine write, wherein the first program pulsecharacteristic is different from the second program pulsecharacteristic.
 5. The system of claim 4, wherein the first programpulse characteristic is different from the second program pulsecharacteristic in one or more of the following ways: a step sizeassociated with the first program pulse characteristic is strictlygreater than a step size associated with the second program pulsecharacteristic, a duration associated with the first program pulsecharacteristic is strictly greater than a duration associated with thesecond program pulse characteristic, or a number of pulses associatedwith the first program pulse characteristic is strictly less than anumber of pulses associated with the second program pulsecharacteristic.
 6. The system of claim 4, wherein the program pulsecharacteristic associated with the Flash memory includes one or more ofthe following: voltage, duration, step size, or a number of pulses. 7.The system of claim 1, wherein: the Flash controller is configured toset the configuration setting of the Flash memory to the first setting,including by: setting a ramp window range associated with the Flashmemory to be a first ramp window range for the accurate coarse write;and the Flash controller is configured to set the configuration settingof the Flash memory to the second setting, including by: setting theramp window range associated with the Flash memory to be a second rampwindow range for the fine write, wherein the first ramp window range isdifferent from the second ramp window range.
 8. The system of claim 7,wherein the first ramp window range is different from the second rampwindow range in one or more of the following ways: a voltage ceiling(v_(H)) associated with the first ramp window range is strictly lessthan a v_(H) associated with the second ramp window range or a voltagefloor (v_(L)) associated with the first ramp window range is strictlyless than a v_(L) associated with the second ramp window range.
 9. Thesystem of claim 1, wherein the Flash controller includes a semiconductordevice, including one or more of the following: an application-specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), anembedded microprocessor, or an Advanced RISC Machine (ARM) embeddedcore.
 10. A computer program product, the computer program product beingembodied in a non-transitory computer readable storage medium andcomprising computer instructions for: receiving an indication to store adata value in a Flash memory; performing an accurate coarse write on theFlash memory, including by: storing a first voltage level in the Flashmemory; and setting a configuration setting of the Flash memory to afirst setting, wherein the first voltage level, when interpreted usingthe configuration setting at the first setting, corresponds to the datavalue; and using the processor to perform a fine write on the Flashmemory, including by: storing a second voltage level in the Flashmemory; and setting the configuration setting of the Flash memory to asecond setting, wherein the second voltage level, when interpreted usingthe configuration setting at the second setting, corresponds to the datavalue.
 11. The computer program product of claim 10, wherein: thecomputer instructions for setting the configuration setting of the Flashmemory to the first setting include computer instructions for setting alevel placement associated with the Flash memory to be a first levelplacement for the accurate coarse write; and the computer instructionsfor setting the configuration setting of the Flash memory to the secondsetting include computer instructions for setting the level placementassociated with the Flash memory to be a second level placement for thefine write, wherein the first level placement is different from thesecond level placement.
 12. The computer program product of claim 11,wherein the first level placement is strictly less than the second levelplacement.
 13. The computer program product of claim 10, wherein: thecomputer instructions for setting the configuration setting of the Flashmemory to the first setting include computer instructions for setting aprogram pulse characteristic associated with the Flash memory to be afirst program pulse characteristic for the accurate coarse write; andthe computer instructions for setting the configuration setting of theFlash memory to the second setting include computer instructions forsetting the program pulse characteristic associated with the Flashmemory to be a second program pulse characteristic for the fine write,wherein the first program pulse characteristic is different from thesecond program pulse characteristic.
 14. The computer program product ofclaim 13, wherein the first program pulse characteristic is differentfrom the second program pulse characteristic in one or more of thefollowing ways: a step size associated with the first program pulsecharacteristic is strictly greater than a step size associated with thesecond program pulse characteristic, a duration associated with thefirst program pulse characteristic is strictly greater than a durationassociated with the second is program pulse characteristic, or a numberof pulses associated with the first program pulse characteristic isstrictly less than a number of pulses associated with the second programpulse characteristic.
 15. The computer program product of claim 13,wherein the program pulse characteristic associated with the Flashmemory includes one or more of the following: voltage, duration, stepsize, or a number of pulses.
 16. The computer program product of claim10, wherein: the computer instructions for setting the configurationsetting of the Flash memory to the first setting include computerinstructions for setting a ramp window range associated with the Flashmemory to be a first ramp window range for the accurate coarse write;and the computer instructions for setting the configuration setting ofthe Flash memory to the second setting include computer instructions forsetting the ramp window range associated with the Flash memory to be asecond ramp window range for the fine write, wherein the first rampwindow range is different from the second ramp window range.
 17. Thecomputer program product of claim 16, wherein the first ramp windowrange is different from the second ramp window range in one or more ofthe following ways: a voltage ceiling (v_(H)) associated with the firstramp window range is strictly less than a v_(H) associated with thesecond ramp window range or a voltage floor (v_(L)) associated with thefirst ramp window range is strictly less than a v_(L) associated withthe second ramp window range.